1. Field of the Invention
The present invention relates generally to the fabrication of glass substrate magnetic recording hard disks, and more particularly to the use of a NiAl seed layer, together with a two part chromium underlayer having a high oxygen concentration portion and a low oxygen concentration portion for depositing the magnetic layer thereon.
2. Description of the Prior Art
The magnetic recording disk in a conventional hard disk drive assembly typically includes a substrate, an underlayer consisting of a thin film of chromium (Cr) or a Cr alloy, a cobalt-based magnetic alloy layer deposited on the underlayer, and a protective overcoat over the magnetic layer. A variety of disk substrates such as NiP-coated AlMg, glass, glass ceramic, glassy carbon, etc., have been used. The microstructural parameters of the magnetic layer, i.e., crystallographic preferred orientation (PO), grain size and magnetic exchange decoupling between the grains, play key roles in controlling the recording characteristics of the disk. The underlayer is mainly used to control such microstructural parameters as the PO and grain size of the cobalt-based magnetic alloy layer. The PO of the various materials forming the layers on the disk is not necessarily an exclusive orientation which may be found in the material, but is merely the dominant orientation. The use of Cr as an underlayer on metallic substrates, such as NiP coated AlMg, typically results in a 11{overscore (2)}0  PO for the magnetic alloy layer. However, since nucleation and growth of Cr or Cr alloy underlayers on glass and most non-metallic substrates differ significantly from those on NiP-coated AlMg substrates, media fabricated on glass substrates often have larger noise compared with those made on NiP-coated AlMg substrates under identical deposition conditions. It is for this reason that the use of an initial layer on the glass substrate (called the seed layer) is necessary. The seed layer is formed between the glass substrate and the underlayer in order to control nucleation and growth of the underlayer, which in turn affects the magnetic layer. Several materials have been proposed for seed layers on glass and non-metallic substrates, such as: Al, Cr, CrNi, Ti, Ni3P, MgO, Ta, C, W, Zr, AlN and NiAl. (See for example, xe2x80x9cSeed Layer induced (002) crystallographic texture in NiAl underlayers,xe2x80x9d Lee, et al., J. Appl. Phys, Vol. 79(8), p.4902ff, Apr. 15, 1996). A glass substrate having a NiAl seed layer and low oxygen Cr underlayer is described by L. Lee et al., IEEE Trans. Magnetic, Vol. 30, Page 3957 (1994). In a glass substrate disk, Laughlin, et al., have described use of an NiAl seed layer followed by a 2.5 nm thick Cr underlayer and a CoCrPt magnetic layer. The NiAl seed layer with the Cr underlayer was said to induce the 10{overscore (1)}0  PO in the magnetic layer. (See xe2x80x9cThe control and Characterization of the Crystallographic Texture of Longitudinal Thin Film Recording Media, IEEE Trans. Magnetic, Vol. 32(5), Page 3632, September 1996).
The improvement in signal to noise ratio (SNR) of the thin film disk media while maintaining a high coercivity remains as one of the major challenges in high density recording technology. A variety of approaches such as choosing a low noise alloy, designing an appropriate underlayer, tailoring of the deposition parameters, and lamination of the magnetic layer have been suggested to reduce the media noise. A chromium underlayer with a high oxygen concentration is known to increase the SNR over a low oxygen concentration underlayer, but the coercivity of the magnetic layer is reduced. In the present invention, an improvement in the underlayer is presented by the controlled introduction of oxygen into the Cr underlayer to create a high oxygen concentration portion followed by a low oxygen concentration portion. As is described hereinbelow, the dual level oxygen concentration Cr underlayer of the present invention both increases SNR and increases the coercivity of the magnetic layer. Thus, control of the oxygen concentration in the Cr underlayer provides control of the magnetic layer coercivity and its SNR.
The thin film magnetic disk of the present invention includes a non-metallic substrate having a seed layer deposited on the substrate, an underlayer deposited upon the seed layer composed of a chromium compound having a relatively high oxygen concentration portion followed by a relatively low oxygen concentration portion, and a magnetic layer that is deposited upon the underlayer. An overcoat is typically, although not necessarily deposited upon the magnetic layer. In the preferred embodiment, the substrate is composed of glass, the seed layer is composed of NiAl, the chromium compound is chromium and the magnetic layer is composed of a cobalt alloy. The underlayer preferably has a high oxygen concentration portion of from 2,000 ppm to 20,000 ppm and preferably approximately 4,000 ppm to 12,000 ppm, and a low oxygen concentration portion of from 0-2,000 ppm, and preferably from 500 ppm to 1,500 ppm. The underlayer total thickness is in the range of from approximately 250 xc3x85 to approximately 700 xc3x85 with a preferred thickness of approximately 450 xc3x85, wherein approximately half of the underlayer thickness is the high oxygen concentration portion and half is the low oxygen concentration portion. The method for manufacturing the present invention utilizes standard thin film deposition techniques, and the underlayer is preferably formed utilizing a target having both chromium and oxygen as target components. In the present invention, the coercivity of the magnetic layer is controlled by adjusting the thickness and oxygen concentration of the underlayer. The coercivity generally decreases with increasing oxygen concentration of the underlayer and increases with increasing thickness of the underlayer. A disk drive of the present invention includes at least one thin film magnetic disk coupled to the spindle of the disk drive.
It is an advantage of the present invention that an improved thin film magnetic disk is provided.
It is a further advantage of the thin film magnetic disk of the present invention that it has a chromium underlayer having a relatively high oxygen concentration portion and a relatively low oxygen concentration portion, which provides improved SNR and coercivity to the magnetic layer of the disk.
It is yet another advantage of the thin film magnetic disk of the present invention that it is manufactured utilizing a process in which the coercivity of the magnetic layer on the disk can be controlled.
It is an advantage of the method of manufacturing the thin film magnetic disk of the present invention that it includes a chromium underlayer with a relatively high oxygen concentration portion and a relatively low oxygen concentration portion that is formed utilizing two targets containing chromium with controlled concentrations of oxygen.
It is another advantage of the method of manufacturing the thin film magnetic disk of the present invention that control over the SNR and coercivity of the magnetic layer is achieved.
These and other features and advantages of the present invention will become known to those skilled in the art upon reviewing the following detailed description which makes reference to the several figures of the drawing.